Same error occurs when inserting other media i.e. music CD, blank DVD etc.

There also seems to be a problem with certain other admin functions for example:
1) Opening "System Settings" then "User Accounts" presents the user screen but the "Unlock" button is grayed out.
2) Using Ubuntu-Tweak, when you access "Source Center" and click "Unlock", nothing happens at all.
3) Shutdown no longer shuts the computer down, it reverts to the Login screen. Using the restart option from here has no effect, only options are to log back in or hit the power button.

I'm using a Toshiba L350 laptop, dual booted from Windows 7. Don't know what else I can do to help but just let me know.

I am suddenly no longer able to mount any of my partitions or external drives / devices. I get the following error message: "Unable to mount XYZ - Not authorized" (where XYZ is the name of the device or partition).

Just noticed I can't either mount any usb devices (micro SD, USB stick, removal hard disk etc) OR shutdown. When I select Shutdown from the menu, the pc drops back to a login box. If I select shutdown from the login box, nothing happens. Restart works ok.

I can confirm the behaviour described by NikTheGeek in comment #25
$ ck-list-sessions shows one active and one inactive session as described by Chris Coulson in comment #8
Updated from Natty to Oneiric, Ubuntu is up to date: lightdm version 1.0.1-0ubuntu6
So maybe a regression?

I forgot to point to a workaround you can use until the bug gets fixed: remove lightdm and (re)install gdm (like in Ubuntu 11.04) as display manager. Open a terminal and enter: 'sudo apt-get remove lightdm' and 'sudo apt-get install gdm'.

I'm running 11.10 with all the latest updates. I'm running the "Gnome classic" desktop, and I still have this bug. I read the whole thread and none of the proposed solutions helped. I didn't have an autologin line in my lightdm.conf, so that didn't help.

Eventually I got too frustrated and switched to gdm and now all is well.

I am also experiencing these problems described by Ian Jefferies on 2011-09-15:
1. Attempting to shutdown fails. Instead, it sends me back to the account login. Thus I can quit only by a physical power off of the PC -- which is pretty hard on it.
2. I am an Administrator, but the User Accounts window shows the unlock icon greyed out, and, indeed, I cannot create a new (second) account.

Problem #1 appeared after I installed (sucessfully) the driver for the Brother HL2207DW laser printer. One mishap during that install was that I used the wrong "wrapper" on my first try, then deleted the wrapper file, and reinstalled with the correct wrapper.

I noticed problem #2 today, but since this was my first attempt to create a second account, I cannot say when it began to manifest.

I should also mention that this problem occurs in spite of my downloading all of the updates, installing & rebooting.

my LightDM version: 1.0.6-0ubuntu1.1
Still has these problems.
If I switch to Guest Session , mounting NTFS partition and USB device will be fine. So maybe there are some configuration left from last 11.04 which is not compatible with current 11.10 version ?
Any infomation we can provide to help ?

I have a slightly different scenario here: I created a new user from the command line as I broke my original user (and that is another problem altogether) and while having fixed most permission problems using vigr, found that I couldn't shut the machine down or mount USB drives. Following the solution from christianwolf above, I looked at /etc/lightdm/lightdm.conf and found that autologin was set to the old username. Commenting that out and rebooting has given me access to the USB drive and I'm yet to test rebooting.

I got the same problems after upgrading to Oneiric (no mount, shutdown, suspend). For me, there wasn't the autologin line in /etc/lightdm/lightdm.conf so that didn't help. I tried downgrading lightdm to 1.0.6-0ubuntu1.1 from 1.2, didn't help. Finally I switched to gdm.

I did the edit of lightdm.conf and that caused problems (no window manager?!?). I then switched to gdm and everything worked. After switching back to lightdm everything still worked. I suspect a problem caused by an update.

If it brakes again, I will try just switching to gdm and back again to see if that fixes things.

I am a first time (first week) linux ubuntu user and this has been extremely frustrating. With zero code knowledge to work with, can somebody instruct me how to dump "lightdm" and get my system working?? No shut down, no user access, no drive access besides the OS drive. I was setting this up for a business that wanted to give it a shot, but so far it's a worse experience than I was hoping for.

#31 (uninstall lightdm install gdm) worked for me. Mine didn't come from the update last fall, rather from creating a second user account and making it "automatic login" (my own administrator account not being automatic login). Deleting the second user account didn't fix the problem, which I suspect may indicate a bug. Perhaps an "Are you sure?" panel in gnome-control-center is in order.

If it ain't broken why the F..K fix it. Locked out to, Can't even do updates!
Love Ubuntu but this goes to far! I am not a tecky so is 10 times harder for me to fix it. I didn't like windows because of the hacking now I don't like thise who program updates just for the sake of issuing an update.
How do I fix this problem with out a 5th belt degree in programming!
I was just looking for the KISS principle in an OS system, maybe time to change again.